Delta-connected, three-phase, four-wire electric panelboards or loadcenters have two 120 nominal volt buses and one 180 volt to 208 volt bus that is commonly referred to as the higher voltage leg or "high leg". (Hereinafter, this bus will be referred to as either the high leg or 208 volt bus, and the lower voltage buses will be referred to as 120 volt buses, even though voltage fluctuates and is not always exactly 120 volts.) Each bus of the distribution panel typically has one-third of the available spaces.
A common problem with this design is that users often fill all of the available 120 volt spaces leaving vacant only the high leg bus spaces. High leg spaces are useful for three-phase motors and other three-phase power equipment, but they cannot be used to power 120 volt equipment. A user who attempts to connect 120 volt equipment to the high leg power supply is likely to burn out the equipment.
A user also cannot double up 120 volt breakers in single breaker pole spaces to provide additional 120 volt to neutral circuits beyond the allotted capacity of a panelboard or loadcenter that derives its power from a delta-connected, four-wire, three-phase power system. Article 384-15 of the National Electric Code prohibits this, and manufacturers design panels or loadcenters not to accept any such excess 120 volt breakers. The consequence of this is that potentially one-third of the available poles or spaces of a delta-connected panelboard or loadcenter are unsuitable for 120 volt to neutral circuits (see FIG. 1). The present solutions are to have any desired additional 120 volt circuits fed from another location or to install a trans former and sub-panel. Unfortunately, these measures are quite costly.
In the end, consumers are disappointed that they are left with vacant spaces in the panel that cannot be used. Electricians are equally frustrated by being unable to offer consumers a cost-effective solution to this problem.